1. Field of the Invention
The present invention relates to an engine exhaust system for automobiles or the like; and more specifically, to an exhaust system having a crook of generally U-shape at the mid section of the pipe line constituting the exhaust system.
2. Description of the Related Art
An engine exhaust system having a crook of approximately U-shape at the mid section of the pipe line constituting the exhaust system as described above is known. For example, when the exhaust pipe extending from the front cylinder of the transverse engine and the exhaust pipe extending from the rear cylinder are linked up at a downstream side of the engine, the exhaust pipe extending from the rear cylinder is curved toward the front below the engine, and then bent into a U-shape to link with the exhaust pipe extending from the front cylinder. Alternatively, there is a case where the exhaust-air flow pipe disposed in the muffler is bent into a U-shape in the muffler for retaining a sufficient length required for achieving a prescribed sound eliminating function.
The cross sectional area of the internal space of the U-shaped crook, including the pipe line in the vicinity thereof, is almost constant; and the pipe line is formed into a required minimum size considering the maximum feed rate of exhaust air and sound eliminating feature. Therefore, the flow of exhaust air is slow at the crook; and thus, resistance is generated and applied to the engine as a back pressure, which may disadvantageously result in lowering of engine performance or engine output.
As a conceivable measure to alleviate resistance at the crook, the cross sectional area of the internal space of the entire pipe line may be increased. However, since such a structure may often cause turbulence or vortex in exhaust air, resistance of flow cannot necessarily be reduced, and in addition, it may disadvantageously lower the sound eliminating capability and increase the cost and the weight.
With such circumstance in view, it is an object of the present invention to provide an engine exhaust system that can minimize flowing resistance of exhaust air even when there is a U-shaped crook at the mid section of the pipe line constituting the exhaust system.
In order to achieve the object described above, an engine exhaust system of the present invention has the following structure.
The engine exhaust system comprising a generally U-shaped crook at the mid section of the pipe line constituting the exhaust system according to the present invention is characterized in that the U-shaped crook forms a gentle curve so that the cross sectional area of the internal space of the U-shaped crook near the top portion is at least 1.3 times the minimum cross sectional area of the internal space of the pipe line having the U-shaped crook.
With such a simple structure that the U-shaped crook forms a gentle curve so that the cross sectional area of the internal space near the top portion of the generally U-shaped crook provided at the mid section of the pipe line constituting the exhaust system is at least 1.3 times the minimum cross sectional area of the internal space of the pipe line having the U-shaped crook, the flowing resistance can be reduced to the minimum value without generating turbulence or vortex in exhaust air in the crook, whereby the lowering of output of the engine due to the back pressure applied to the engine can conveniently be prevented. In addition, since the cross sectional area of the internal space of the generally straight pipe portion on both sides of the crook may be the minimum size, the cost of materials or the weight can be reduced in comparison with the case where the diameter of the entire crook including the straight pipe portion is large as in the related art; and thus, it can conveniently be arranged within a narrow space such as the bottom portion of the vehicle body or the muffler.